Birefringent marking

ABSTRACT

The invention relates to a method of preparing a birefringent marking by printing a polymerisable liquid crystal material onto at least one side of a substrate and polymerising the liquid crystal material, to a birefringent marking obtained by this method, and to the use of the birefringent marking for decorative, security, authentification and identification applications.

FIELD OF THE INVENTION

The invention relates to a method of preparing a birefringent markingcomprising polymerised liquid crystal material by applying apolymerisable liquid crystal material onto at least one side of asubstrate using printing techniques and polymerising the liquid crystalmaterial. The invention further relates to a birefringent markingobtained by this method, and to the use of the birefringent marking fordecorative, security, authentification or identification applications.Furthermore, the invention relates to a security, authentification oridentification marking, thread or device, to a document of value, to ahot stamping foil, to a reflective foil and to an optical data storagedevice comprising the birefringent marking according to the invention.

BACKGROUND AND PRIOR ART

Birefringent films comprising polymer liquid crystal material are knownin prior art. GB 2 324 382 discloses a liquid crystal polymer film withhomeotropic alignment prepared from a polymerisable liquid crystalmaterial that is coated onto a plastic substrate and polymerised,whereby roll to roll coating is applicable. The use of an aligning layercomprising a surfactant being fixed in a matrix of a polymeric liquidcrystalline material is also described. According to an alternativeembodiment the substrate is coated with aluminium or sufficiently smoothAl₂O₃ to achieve a homeotropic alignment of the liquid crystal material.The resulting uniformly homeotropic aligned films can be used inelectrooptical displays, optical compensation layers or as activeswitching layer.

The use of birefringent films as security devices has also been reportedin prior art.

GB 2 330 360 describes a polymer film with a helically twisted molecularstructure exhibiting at least two maxima of a reflection wavelength. Thefilm is obtained by polymerizing a thermochromic mesogenic compositionat varied temperatures. Since the temperature is changed duringpolymerization, the thermochromic polymerizable composition undergoes achange of the helical pitch and therewith of the reflection maximum. Thevarious pitch lengths are fixed by the polymerization reaction and apolymer film is obtained that exhibits, depending on the type, degreeand speed of temperature variation, a range of reflection colours.

However, a disadvantage of the method described in GB 2 330 360 is therequirement of a precise control of the temperature and its changeduring the polymerization process, and the need of specific materialswith thermochromic properties.

GB 2 357 061 discloses a hot stamping foil for security applicationscomprising a layer of polymerised liquid crystal material. The liquidcrystal polymer layer is prepared on a plastic substrate and thenlaminated with a reflective layer of e.g. aluminium which exhibits onits opposite surface a hot melt adhesive. The liquid crystal layer isprotected by a lacquer. The whole arrangement of layers is carried by asupport layer. A wax layer between the support layer and the lacquerlayer enables a release of the birefringent layer arrangement byapplying heat. The liquid crystal material is for example a polymerisedor crosslinked nematic or smectic material with a planar, tilted,splayed or homeotropic structure and macroscopically uniformorientation. The hot stamping foil can be used for decorative purposesor to authenticate and prevent counterfeiting of documents of value,e.g. of banknotes, credit cards or ID cards. The transmission orreflection effect of the polarized light by the birefringent materialcan be seen using linear polarizers. Incorporation of dichroic dyesleads to additional color effects.

However, the process described in GB 2 357 061 requires the additionalprocess step of transferring or laminating the polymeric liquidcrystalline material, which is typically prepared on a plasticsubstrate, to the reflective aluminium layer, or alternatively applyingthe aluminium layer onto the liquid crystal layer. This is requiredsince the natural alignment of nematic liquid crystals on aluminiumsurfaces is homeotropic. However, this alignment would not be useful fordisplaying hidden designs. Also, the preparation of a liquid crystalpolymer layer by coating a polymerisable material onto a substrateusually yields uniform coated areas of the material. The formation ofimages or patterns would require specific materials or additionaltechniquies such as photomasking or photoorientation.

AIMS OF THE INVENTION

The aim of the present invention is to provide a birefringent marking,in particular for decorative, security, authentification oridentification applications, that does not have the drawbacks of theprior art devices, exhibits effects difficult to counterfeit and allowsan economic fabrication even at large scales.

A further aim of this invention relates to a method of preparing abirefringent marking according to this invention, which allows a fast,reliable and cheap fabrication.

A further aim of this invention is to provide an advantageous use of thebirefringent marking according to this invention.

Further aims of this invention relate to security, authentification oridentification markings or devices, to a reflective film and to anoptical data storage device, which are difficult to counterfeit.

Other aims of the present invention are immediately evident to theperson skilled in the art from the following detailed description.

The inventors have found that the above aims can be achieved byproviding a birefringent marking comprising polymerised liquid crystalmaterial obtained by a method according to the present invention. Insaid method a polymerisable liquid crystal material is applied onto asubstrate and polymerised, wherein the substrate is a reflectivesubstrate and the polymerisable liquid crystal material is applied byusing printing techniques. A birefringent marking prepared by saidmethod shows particularly striking visual effects.

SUMMARY OF THE INVENTION

The invention relates to a method of preparing a birefringent markingcomprising polymerised liquid crystal material by printing apolymerisable liquid crystal material onto at least one surface of areflective substrate and polymerising the liquid crystal material.

The present invention further relates to a birefringent markingobtainable by a method according to the present invention.

The invention further relates to the use of a birefringent markingaccording to the present invention in decorative, security,authentification or identification applications.

The invention further relates to a security, authentification oridentification marking, thread or device comprising at least onebirefringent marking according to the present invention.

The invention further relates to a document of value, hot stamping foil,reflective foil or optical data storage device comprising at least onebirefringent marking or at least one security, authentification oridentification marking, thread or device according to the presentinvention.

DEFINITION OF TERMS

In connection with liquid crystal layers and films as described in thepresent application, the following definitions of terms as usedthroughout this application are given.

The term ‘film’ as used in this application includes self-supporting,i.e. free-standing, films or foils that show more or less pronouncedmechanical stability and flexibility, as well as precoated, preprintedor laminated foils whereas the coating or printing can be partially orcompletely, as well as coatings or layers on a supporting substrate orbetween two or more substrates.

The term ‘marking’ includes films or coatings or layers covering theentire area of a substrate, as well as markings covering discreteregions of a substrate for example in the shape of a regular pattern orimage.

The term ‘liquid crystal or mesogenic material’ or ‘liquid crystal ormesogenic compound’ should denote materials or compounds comprising oneor more rod-shaped, board-shaped or disk-shaped mesogenic groups, i.e.groups with the ability to induce liquid crystal phase behaviour. Liquidcrystal compounds with rod-shaped or board-shaped groups are also knownin the art as ‘calamitic’ liquid crystals. Liquid crystal compounds witha disk-shaped group are also known in the art as ‘discotic’ liquidcrystals. The compounds or materials comprising mesogenic groups do notnecessarily have to exhibit a liquid crystal phase themselves. It isalso possible that they show liquid crystal phase behaviour only inmixtures with other compounds, or when the mesogenic compounds ormaterials, or the mixtures thereof, are polymerized.

For the sake of simplicity, the term ‘liquid crystal material’ is usedhereinafter for both liquid crystal materials and mesogenic materials,and the term ‘mesogen’ is used for the mesogenic groups of the material.

The term ‘non-chiral’ material includes materials that consistexclusively of achiral compounds, as well as materials that consist ofor comprise a racemate.

The director means the preferred orientation direction of the longmolecular axes (in case of calamitic compounds) or short molecular axis(in case of discoic compounds) of the mesogens in a liquid crystalmaterial.

The term ‘planar structure’, ‘planar alignment’ or ‘planar orientation’refers to a layer or film of liquid crystal material wherein thedirector is substantially parallel to the plane of the film or layer.

The term ‘homeotropic structure’, ‘homeotropic alignment’ or‘homeotropic orientation’ refers to a layer or film of liquid crystalmaterial wherein the director is substantially perpendicular to the filmplane, i.e. substantially parallel to the film normal.

The term ‘tilted structure’, ‘tilted alignment’ or ‘tilted orientation’refers to a layer or film of liquid crystal material wherein thedirector is tilted at an angle θ of between 0 and 90 degrees relative tothe film plane.

The term ‘splayed structure’, ‘splayed alignment’ or ‘splayedorientation’ means a tilted orientation as defined above, wherein thetilt angle varies monotonuously in the range from 0 to 90°, preferablyfrom a minimum to a maximum value, in a direction perpendicular to thefilm plane.

For sake of simplicity, a film comprising liquid crystal material with aplanar, homeotropic, tilted or splayed orientation, alignment orstructure is hereinafter also referred to as ‘planar film’, ‘homeotropicfilm’, ‘tilted film’ and ‘splayed film’, respectively.

The term “reflectve substrate” covers substrates with mirrorlikesurfaces for printing onto metal films, substrates showing Lambertianreflection which are especially suitable when printing onto e.g.pearlescent pigment systems, and substrates that comprise or are part ofan optically variable device (OVD), like for example a diffractiongrating, hologram or kinegram. “Reflection” means reflection of lightinside the visible range of the spectrum (with wavelengths fromapproximately 400 to 800 nm) and outside the visible range, e.g. in theUV or IR range (i.e. with wavelengths of less than 400 nm or more than800 nm).

DETAILED DESCRIPTION OF THE INVENTION

Preferred embodiments of the invention relate to a method orbirefringent marking wherein

-   -   the liquid crystal material is a non-chiral material,    -   the liquid crystal material does not comprise chiral compounds,    -   the liquid crystal material comprises a racemate,    -   the liquid crystal material does not have a helically twisted        structure,    -   the liquid crystal material does not show selective reflection        of wavelengths in or below the IR range,    -   the polymerisable liquid crystal material is a nematic or        smectic liquid crystal material,    -   the reflective substrate comprises at least one metallic or        metallised layer, wherein the metal is preferably selected from        aluminium, gold and copper, very preferably aluminium,    -   the reflective substrate is a metallic film, a metallised paper        or polymer film, a hot stamping foil or a holographic image,    -   the reflective substrate comprises at least one layer of        reflective pigments, preferably selected from interference or        pearlescent pigments or liquid crystal pigments.    -   the reflective substrate comprises at least one layer of        dielectric coating for reflection of a specific range of        electromagnetic spectra.    -   the reflective substrate is a holographic optical element (HOE)        for reflection of a specific range of electromagnetic spectra.    -   the reflective substrate is a hologram optimized for reflection        of a specific range of electromagnetic spectra or a rainbow        hologram.    -   the liquid crystal material comprises at least one compound        which induces and/or enhances a planar alignment,    -   the compound inducing and/or enhancing a planar alignment is a        surfactant, preferably a nonionic surfactant, the polymerised        liquid crystal material has a planar structure,    -   the polymerised liquid crystal material has a splayed structure.

The reflective substrate can be a substrate showing specular or Braggreflection or an OVD (optically variable device) layer or a part of anOVD layer. Preferred reflective substrates are metallic or metallisedsubstrates, i.e. substrates incorporating or being covered by one ormore metal layers. In addition these substrates may also be part of anOVD, like a hot stamping foil or a holographic image. Further preferredare substrates incorporating or being covered by one or more layers ofreflective pigments, like metal flakes, pearlescent or interferencepigments or liquid crystal pigments, or mixtures thereof.

Metal substrates or metallised layers can be selected e.g of Al, Cu, Ni,Ag, Cr or alloys like e.g. Pt—Rh or Ni—Cr, or layers comprising one ormore metal flakes dispersed in a light transmissive binder. Suitablemetal flakes are e.g. flakes aluminium, gold or titan, or metal oxideflakes of e.g. Fe₂O₃ and/or TiO₂.

Suitable pearlescent or interference pigments are e.g. mica, SiO₂,Al₂O₃, TiO₂ or glass flakes that are coated with one or more layers ofe.g. titanium dioxide, iron oxide, titanium iron oxide or chrome oxideor combinations thereof, flakes comprising combinations of metal andmetal oxide, metal flakes of e.g. aluminium coated with layers of ironoxide layers and/or silicium dioxide. These pigments are known to theexpert and are commercially available in a wide variety. Preferredpigments are for example the commercially available Iriodin®,Colourstream® or Xirallic® (from Merck KGaA, Darmstadt, Germany), orPaliochrome® (from BASF AG, Ludwigshafen, Germany), or opticallyvariable pigments e.g. from Flex Corp.

It is also possible to use liquid crystal pigments or coatingscomprising a polymerized or crosslinked cholesteric liquid crystalmaterial, e.g. cholesteric liquid crystal pigments dispersed in atransparent binder. Suitable liquid crystal pigments and binder systemsare known to the expert and are described for example in U.S. Pat. Nos.5,364,557, 5,834,072, EP 0 601 483, WO 94/22976, WO 97/27251, WO97/27252, WO 97/30136 or WO 99/02340.

Preferably the substrate comprises a surface of metal, in particular ofaluminium, at the substrate—liquid crystal layer interface.

Further preferred are substrates covered by one or more layers ofpearlescent or intereference pigments or liquid crystal pigments, ormixtures thereof, which are dispersed in a transparent binder.

Suitable substrates include films, paper, board, leather, cellulosesheeting, textiles, plastics, glass, ceramics and metals. Suitablepolymer films are for example polyester such aspolyethyleneterephthalate (PET) or polyethylenenaphthalate (PEN),polyvinylalcohol (PVA), polycarbonate (PC) or triacetylcellulose (TAC),especially preferably PET or TAC. Especially preferred are substratesmetallised with aluminium, or aluminium foils.

The substrate or at least the surface of the substrate printed with theliquid crystal material is preferably flat, like e.g. a foil, film orsheet, and has preferably a thickness smaller than 200 μm, in particularsmaller than 60 μm, most preferably smaller than 20 μm.

The substrate surface is preferably planar, but may also be structured,patterned and/or have a relief. The shape, structure, pattern and/orrelief of the substrate is preferably adapted to the desired applicationof the inventive birefringent marking. Suitable structuring andpatterning techniques are well known to the one skilled in the art, inparticular in the fields of precision engineering and microtechnology,and include but are not limited to lithography, etching, cutting,stamping, punching, embossing, molding and electron discharge machiningtechniques.

It is also possible to use a reflective substrate comprising or beingpart of an OVD, like a diffraction grating, hologram, kinegram or commonholographic optical element (HOE), a holographic layer with an embossed,patterned or structured surface, or a layer of reflective holographicpigments. Light reflected by higher regions of the structured surfacewill interfer with light reflected by lower regions of the structuredsurface, thereby forming a holographic image. The preparation ofholographic layers is described for example in U.S. Pat. No. 4,588,664,the entire disclosure of which is incorporated into this application byreference.

Thus, for example a substrate like e.g. a banknote, or selected regionsthereof, can have applied to it a hologram or reflective metal layer,onto which the liquid crystal material is printed. Alternatively themarking may be prepared separately on a reflective substrate which isthen applied to the document of value for example as security thread oras another form of a security marking.

This embodiment is particularly suitable for use as false-proof securitythreads or holograms on banknotes or documents of value, providing asecurity marking by which the banknote is easy to authenticate whenviewed through a polariser.

The liquid crystal material may be applied onto one side or on bothsides of the reflective substrate. It may be printed onto discreteregions of the substrate to form a pattern or image that is for examplevisible under unpolarised light due to a change in the surface gloss inthe printed areas, or may be invisible under unpolarised light andbecome visible only when viewed through a polariser. Alternatively theliquid crystal material may be printed onto the entire substrate to forma continuous layer or film that becomes visible only when viewed througha polariser.

Preferably the birefringent marking is prepared by printing a mixturecomprising the liquid crystal material and a solvent onto the reflectivesubstrate and allowing the solvent to evaporate before or duringpolymerisation. The mixture can be a solution, dispersion or emulsion ofthe liquid crystal material in the solvent. Preferably a solution isused. The solvent may evaporate with or without the use of externalforces such as heat or reduced pressure.

The liquid crystal material can be printed onto the substate byconventional printing techniques which are known to the expert,including for example screen printing, offset printing, dry offsetprinting reel-to-reel printing, letter press printing, gravure printing,rotogravure printing, flexographic printing, intaglio printing, padprinting, heat-seal printing, ink-jet printing, thermal transferprinting or printing by means of a stamp or printing plate. The printingprocess induces or enhances spontaneous alignment of the liquid crystalmaterial on the substrate.

The liquid crystal material may additionally comprise a polymeric binderor one or more monomers capable of forming a polymeric binder and/or oneor more dispersion auxiliaries. Suitable binders and dispersionauxiliaries are disclosed for example in WO 96/02597.

Especially preferred are liquid crystal materials not containing abinder or dispersion auxiliary.

In a preferred embodiment the liquid crystal material comprises anadditive that induces or enhances planar alignment of the liquid crystalmaterial on the substrate. Preferably the additive comprises one or moresurfactants. Suitable surfactants are described for example in J.Cognard, Mol. Cryst. Liq. Cryst. 78, Supplement 1, 1-77 (1981).Particularly preferred are non-ionic surfactants, very, fluorocarbonsurfactants, like for example the commercially available fluorocarbonsurfactants Fluorad FC-171® (from 3M Co.), or Zonyl FSN® (from DuPont).

Suitable and preferred fluorocarbons surfactant are for example those offormula IC_(n)F_(2n+1)SO₂N(C₂H₅)(CH₂CH₂O)_(x)CH₃  Iwherein n is an integer from 4 to 12 and x is an integer from 5 to 15,which are commercially available as Fluorad FC-171® (from 3M Co.).

The lower limit of the amount of the additives inducing planar alignmentin the polymerisable liquid crystal material is preferably 0.01 weight%, in particular 0.05 weight %, most preferably 0.1 weight % of theliquid crystal material. The upper limit of the amount of said compoundsis preferably 5 weight %, in particular 3 weight %, most preferably 1.5weight % of the liquid crystal material.

The optimum range of the thickness of the birefringent marking dependsupon the strength of the birefringence of the liquid crystal material.The person skilled in the art can determine easily the optimum range ofthe thickness. Birefringent markings with a birefringence in the rangefrom 0.1 to 0.3 generally have a preferred thickness from 0.5 to 20 μm,especially 0.7 to 10 μm, most preferably 1 to 6 μm.

The thickness of the marking can be altered for example by changing thesolvent concentration of the liquid crystal mixture. The thickness ofthe final liquid crystal layer thus increases with increasing amount ofliquid crystal material in the mixture. Furthermore a wetting agent or asurfactant may be added to the liquid crystal solution to change itssurface tension and its adhesion to the substrate. Such a wetting agentor surfactant may be the same surfactant or be added to the surfactantwhich is used to induce a planar alignment as described above. Suitablesurfactants are those mentioned above.

The birefringent marking according to this invention may comprise one ormore further layers, which are preferably applied onto the birefringentlayer.

The further layers may be chosen according to the desired application ofthe birefringent film and may have a function, like e.g. as a protectinglayer, support layer, adhesive layer, reflecting layer, opticalretardation layer, colour filter and/or as a polariser.

For example, a preferred marking according to the invention comprises acircular or linear polariser applied on top of the liquid crystal layer.Thereby the marking becomes permanently visible and can be used e.g. asvisible verification or copy-protection marking on an article or adocument of value.

Suitable linear polarisers are known in the art. For exampleconventional linear absorption polarisers can be used, which typicallycomprise an uniaxially stretched polymer film of e.g. polyvinyl alcohol,or a polymer film into which a dichroic dye is incorporated. It is alsopossible to use a linear polariser comprising a layer of vitrified,polymerised or crosslinked liquid crystal (LC) material with planarstructure, as described for example in EP-A-0 397 263, the entiredisclosure of which is incorporated into this application by way ofreference.

Suitable circular polarisers are also known in the art. For example acombination of a conventional linear polariser and a quarter waveretardation film (QWF) can be used. Alternatively it is possible to usea circular polariser comprising a layer of vitrified, polymerised orcrosslinked cholesteric liquid crystal (LC) material with planarstructure, which can operate as reflective polariser or colour filter ina narrow wavelength band (notch polariser), or in a broad wavelengthband, as described for example in EP 0 606 940, WO 97/35219, EP 0 982605 and WO 99/02340, the entire disclosure of which is incorporated intothis application by reference.

Another preferred marking according to the invention comprises one ormore optical retardation layer phase causing an optical phase shift ofthe light transmitted by the liquid crystal material and reflected bythe reflective substrate. This leads to an additional colour shift ofthe birefringent marking when viewed through a polariser. Preferably,the retardation layer is a quarter wave film or foil (QWF) exhibiting anet retardation that is approximately 0.25 times the wavelengthtransmitted by the circular polarising layer.

Suitable retardation layers are known in the art and typically compriseuniaxially or biaxially stretched or compressed films of an isotropicpolymer, like e.g. polyethylene terephthalate (PET), polyvinyl alcohol(PVA), polycarbonate (PC) or di- or triacetyl cellulose (DAC, TAC). Itis also possible to use a phase shift layer or retardation filmcomprising a layer of vitrified, polymerised or crosslinked liquidcrystalline material with planar structure, as described for example inWO 98/04651, or comprising a layer of vitrified, polymerised orcrosslinked liquid crystalline material with tilted or splayedstructure, as described for example in WO 98/12584, the entiredisclosure of which is incorporated into this application by way ofreference.

According to a first preferred embodiment of the invention, a nematic orsmectic, preferably a nematic polymerisable liquid crystal material,comprising a surfactant is printed onto discrete regions of a metallisedor metal substrate and the material in the printed regions ispolymerised in-situ.

When viewing this marking under unpolarised light no colour effect isseen. The liquid crystal material is transparent and visible to thenaked eye only at shorter distances due to the change of surface glossin the printed regions. By covering the printed regions of thesubstrate, or the entire substrate including the printed and thenon-printed regions, with a transparent lacquer, the change of surfacegloss between the printed and non-printed regions can be prevented andthe marking can be made completely invisible when viewed underunpolarised light. In case the marking is still visible under a specificviewing angle by its different thickness, this can be overcome byapplying the negative marking with an ink containing no LC material. Alater varnishing of the complete surface can enhance this camouflage.

When viewing this marking through a linear polariser against thereflective background a colour can be seen in the printed regions atspecific angle intervals as the polariser is rotated. The strength ofthe colour can be influenced by varying the type of the reflectivesubstrate.

When viewing the marking through a circular polariser the printedregions are visible against a darker background.

According to a second preferred embodiment of the invention, a nematicor smectic, preferably a nematic polymerisable liquid crystal material,comprising a surfactant is printed onto discrete regions of the surfaceof e.g. a paper substrate that is covered by a layer of interferencepigments dispersed in a transparent binder, and the liquid crystalmaterial in the printed regions is polymerised in-situ.

When viewing this marking under unpolarised light no colour effect isseen. The liquid crystal material is transparent and visible to thenaked eye only at shorter distances due to the change of surface glossin the printed areas. By covering the printed regions of the substrate,or the entire substrate including the printed and the non-printedregions, with a transparent lacquer, the change of surface gloss betweenthe printed and non-printed regions can be prevented and the marking canbe made completely invisible when viewed under unpolarised light.

When viewing this marking through a linear polariser against thereflective background the following effect is observed. The printedregions appear as slightly darker areas against a lighter background. Incases where the alignment of the liquid crystal material is planar thenthe printed areas will become darker, at specific angles of thepolariser, as the polariser is rotated.

When viewing the marking through a circular polariser the printedregions appear visible against a darker background. This effect isindependent of the orientation of the circular polariser.

The above effects cannot be achieved with standard security markings.Thus the birefringent marking according to the invention is especiallysuited for security, authentification or identification markings thatcannot be counterfeited or copied easily, but also for decorativepurposes. Furthermore, the birefringent marking can be fabricatedeconomically even at large scales using printing processes. The methodaccording to the present invention allows a fast, reliable and cheapfabrication of the inventive birefringent marking.

According to another preferred embodiment of the invention, theinventive birefringent film is part of a hot stamping foil. Thus a hotstamping foil comprising the inventive birefringent film is also anobject of this invention. Preferred structures and the assembly of hotstamping foils are described in the GB 2 357 061, which is incorporatedherein by reference in its entirety. The reflective substrate of thebirefringent film according to the preferred embodiment of thisinvention corresponds to the reflective layer described in the GB 2 357061, onto which the layer of liquid crystalline material is printed.

According to another preferred embodiment of the invention, theinventive birefringent marking is part of an optical data storagedevice. Thus an optical data storage device comprising the inventivebirefringent film is also an object of this invention. As the inventivebirefringent film does not affect the optical function, it is especiallysuited to mark such devices, in particular compact discs (CD), which maybe read and/or write memory devices for data, like software, audioand/or video data.

In general such compact discs comprise a disc, being made of atransparent plastic material, like polycarbonate. One side of theplastic contains the data in digitized form (data pits) and ismetallised, e.g. by a thin aluminium coating, which acts as a reflectivesurface for the laserbeam and is usually sputtered directly on top ofthe data pits. On top of the aluminum layer a protective varnish isspin-coated. On top of the protective varnish a printed image can beapplied. The reading of the data is done through the transparent plasticmaterial. According to one variant of this embodiment, the inventivebirefringent marking can be directly printed onto the protectivevarnish, which serves as a substrate. The birefringent marking may coverone side of the disc completely or only one or more parts of it. Whenviewing from the printed side of the disc through a linear polarizer onecan see the non-regular pattern as described.

The LC material of the birefringent layer is preferably a nematic orsmectic LC material. Nematic LC materials are especially preferred.

Preferably the liquid crystal material is a polymerisable orcrosslinkable material, preferably dissolved in an organic solvent,which is polymerised or crosslinked by in-situ polymerisation during orafter evaporation of the solvent.

The polymerisable liquid crystal material preferably comprises at leastone monoreactive achiral polymerisable mesogenic compound and at leastone di- or multireactive achiral polymerisable mesogenic compound.

If di- or multireactive compounds are present in the polymerisablematerial, a three-dimensional polymer network is formed and the planarorientation of the LC material is permanently fixed. An polymer filmmade of such a network is self-supporting and shows a high mechanicaland thermal stability and a low temperature dependence of its physicaland optical properties.

By varying the concentration of the di- and multireactive compounds thecrosslink density of the polymer film and thereby its physical andchemical properties such as the glass transition temperature, which isalso important for the temperature dependence of the optical propertiesof the optical retardation film, the thermal and mechanical stability orthe solvent resistance can be tuned easily.

A preferred polymerisable liquid crystal material comprises

-   -   5-60% of one or more direactive achiral mesogenic compounds,    -   20-90% of one or more monoreactive achiral mesogenic compounds,    -   0 to 10%, preferably 0.1 to 5% of one or more surfactants,    -   0 to 10%, preferably 0.1 to 5% of one or more photoinitiators.

The polymerisable compounds and polymerisable mesogenic compoundsreferred to above and below are preferably monomers.

The polymerisable mesogenic mono-, di- or multireactive compounds usedfor the instant invention can be prepared by methods which are known perse and which are described, for example, in standard works of organicchemistry such as, for example, Houben-Weyl, Methoden der organischenChemie, Thieme-Verlag, Stuttgart. Typical examples are described forexample in WO 93/22397; EP 0 261 712; DE 19504224; DE 4408171 and DE4405316. The compounds disclosed in these documents, however, are to beregarded merely as examples that do not limit the scope of thisinvention.

Examples representing especially useful mono- and direactivepolymerisable mesogenic compounds are shown in the following list ofcompounds, which should, however, be taken only as illustrative and isin no way intended to restrict, but instead to explain the presentinvention:

In the above formulae, P is a polymerisable group, preferably an acryl,methacryl, vinyl, vinyloxy, propenyl ether, epoxy or styryl group, x andy are each independently 1 to 12, A is 1,4-phenylene that is optionallymono di or trisubstituted by L¹ or 1,4-cyclohexylene, v is 0 or 1, Z⁰ is—COO—, —OCO—, —CH₂CH₂—, —C≡C— or a single bond, Y is a polar group, R⁰is an non-polar alkyl or alkoxy group, and L¹ and L² are eachindependently H, F, Cl, CN or an optionally halogenated alkyl, alkoxy,alkylcarbonyl, alkoxycarbonyl or alkoxycarbonyloxy group with 1 to 7 Catoms.

The term ‘polar group’ in this connection means a group selected from F,Cl, CN, NO₂, OH, OCH₃, OCN, SCN, an optionally fluorinated carbonyl orcarboxyl group with up to 4 C atoms or a mono- oligo- or polyfluorinatedalkyl or alkoxy group with 1 to 4 C atoms. The term ‘non-polar group’means an alkyl group with 1 or more, preferably 1 to 12 C atoms or analkoxy group with 2 or more, preferably 2 to 12 C atoms.

Polymerisation of the polymerisable LC material can be achieved forexample by exposing it to heat or actinic radiation. Actinic radiationmeans irradiation with light, like UV light, IR light or visible light,irradiation with X-rays or gamma rays or irradiation with high energyparticles, such as ions or electrons. Preferably polymerisation iscarried out by UV irradiation. As a source for actinic radiation forexample a single UV lamp or a set of UV lamps can be used. When using ahigh lamp power the curing time can be reduced. Another possible sourcefor actinic radiation is a laser, like e.g. a UV laser, an IR laser or avisible laser.

The polymerisation is preferably carried out in the presence of aninitiator absorbing at the wavelength of the actinic radiation. Forexample, when polymerising by means of UV light, a photoinitiator can beused that decomposes under UV irradiation to produce free radicals orions that start the polymerisation reaction. When curing polymerisablemesogens with acrylate or methacrylate groups, preferably a radicalphotoinitiator is used, when curing polymerisable mesogens with vinyland epoxide groups, preferably a cationic photoinitiator is used. It isalso possible to use a polymerisation initiator that decomposes whenheated to produce free radicals or ions that start the polymerisation.As a photoinitiator for radical polymerisation for example thecommercially available Irgacure 651, Irgacure 184, Darocure 1173 orDarocure 4205 (all from Ciba Geigy AG) can be used, whereas in case ofcationic photopolymerisation the commercially available UVI 6974 (UnionCarbide) can be used. The polymerisable LC material preferably comprises0.01 to 10%, very preferably 0.05 to 5%, in particular 0.1 to 3% of apolymerisation initiator. UV photoinitiators are preferred, inparticular radicalic UV photoinitiators.

The curing time is dependent, inter alia, on the reactivity of thepolymerisable mesogenic material, the thickness of the printed layer,the type of polymerisation initiator and the power of the UV lamp. Thecuring time according to the invention is preferably not longer than 10minutes, particularly preferably not longer than 5 minutes and veryparticularly preferably shorter than 2 minutes. For mass productionshort curing times of 3 minutes or less, very preferably of 1 minute orless, in particular of 30 seconds or less, are preferred.

The inventive polymerisable liquid crystalline mixtures can additionallycomprise one or more other suitable components or additives such as, forexample, catalysts, sensitizers, stabilizers, inhibitors, co-reactingmonomers, surface-active compounds, lubricating agents, wetting agents,dispersing agents, hydrophobing agents, adhesive agents, flow improvers,defoaming agents, deaerators, inert diluents, reactive diluents,auxiliaries, colourants, dyes or pigments.

Suitable additives are disclosed for example in WO 00/47694, the entiredisclosure of which is incorporated into this application by reference.

In particular the addition of stabilizers is preferred in order toprevent undesired spontaneous polymerisation of the polymerisablematerial for example during storage. As stabilizers in principal allcompounds can be used that are known to the skilled in the art for thispurpose.

These compounds are commercially available in a broad variety. Typicalexamples for stabilizers are 4-ethoxyphenol or butylated hydroxytoluene(BHT).

Other additives, like e.g. chain transfer agents, can also be added tothe polymerisable LC material in order to modify the physical propertiesof the resulting polymer film. When adding a chain transfer agent, suchas monofunctional thiol compounds like e.g. dodecane thiol ormultifunctional thiol compounds like e.g. trimethylpropanetri(3-mercaptopropionate), to the polymerisable material, the length ofthe free polymer chains and/or the length of the polymer chains betweentwo crosslinks in the inventive polymer film can be controlled. When theamount of the chain transfer agent is increased, the polymer chainlength in the obtained polymer film is decreasing.

It is also possible, in order to increase crosslinking of the polymers,to add up to 20% of a non mesogenic compound with two or morepolymerisable functional groups to the polymerisable LC materialalternatively or in addition to the di- or multifunctional polymerisablemesogenic compounds to increase crosslinking of the polymer. Typicalexamples for difunctional non mesogenic monomers are alkyldiacrylates oralkyldimethacrylates with alkyl groups of 1 to 20 C atoms. Typicalexamples for non mesogenic monomers with more than two polymerisablegroups are trimethylpropanetrimethacrylate orpentaerythritoltetraacrylate.

In another preferred embodiment the mixture of polymerisable materialcomprises up to 70%, preferably 3 to 50% of a non mesogenic compoundwith one polymerisable functional group. Typical examples formonofunctional non mesogenic monomers are alkylacrylates oralkylmethacrylates.

It is also possible to add, for example, a quantity of up to 20%/o byweight of a non polymerisable liquid-crystalline compound to adapt theoptical properties of the resulting polymer film.

The polymerisation is preferably carried out in the liquid crystal phaseof the polymerisable LC material. Therefore, preferably polymerisablemesogenic compounds or mixtures with low melting points and broad liquidcrystal phase ranges are used. These types of materials do not requirehigh temperatures over a long period to generate the correct phase andso the use of such materials allows the reduction of the polymerisationtemperature, which makes the polymerisation process from a technicalpoint of view easier and is a considerable advantage for massproduction. The selection of suitable polymerisation temperaturesdepends mainly on the clearing point of the polymerisable material andinter alia on the softening point of the substrate. Preferably thepolymerisation temperature is at least 30 degrees below the clearingtemperature of the polymerisable mesogenic mixture. Polymerisationtemperatures below 120° C. are preferred. Especially preferred aretemperatures below 90° C., in particular temperatures of 60° C. or less.

The inventive birefringent marking according to this invention can beused in decorative, security, authentification or identificationapplications, as security, authentification or identification marking,or in a thread or device comprising the birefringent marking.

The birefringent marking can be used in various ways. For example, it ispossible to prepare a metallic thread with a polymerised liquid crystalmaterial as described above and then apply this thread to a securitydocument, either as part of a hot stamping foil (HSF) or as a woventhread. According to a further embodiment the liquid crystal material isapplied directly to a reflecting area on an existing security document.e.g. overprinting the reflecting area on a banknote.

The birefringent marking can be used for direct application e.g. onto anarticle, device or document, or as threads, holograms or hot stampingfoils for decorative or security applications, to authenticate andprevent counterfeiting of documents of value, for identification ofhidden images, informations or patterns. It can be applied to consumerproducts or household objects, car bodies, foils, packing materials,clothes or woven fabric, incorporated into plastic, or applied assecurity markings or threads on documents of value like banknotes,credit cards or ID cards, national ID documents, licenses or any productwith money value, like stamps, tickets, shares, cheques etc.

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description, utilize the present invention toits fullest extent. The following examples are, therefore, to beconstrued as merely illustrative and not limitative of the remainder ofthe disclosure in any way whatsoever.

In the foregoing and in the following examples, unless otherwiseindicated, all temperatures are set forth uncorrected in degrees Celsiusand all parts and percentages are by weight.

EXAMPLE 1

The following polymerisable mixture is prepared

Compound (A) 12.12% Compound (B) 29.66% Compound (C) 22.90% Compound (D)28.37% Irgacure 907  6.42% Fluorad FC 171  0.53% (A)

(B)

(C)

(D)

Compounds (A), (B) and (C) can be prepared according to or in analogy tothe methods described in D. J. Broer et al., Makromol. Chem. 190,3201-3215 (1989). Compound (D) and its preparation are described in GB2,280,445. Irgacure 907 is a commercially available photoinitiator (CibaGeigy). Fluorad FC 171 is a commercially available surfactant (3M Inc.).

A 30 weight-% solution of the polymerisable mixture in toluene wasprepared and filtered through a 1 μm filter. The solution was thenplaced in the ink chamber of a flexographic printing machine andtransferred to an Anilox roller. From this roller the solution wastransferred to a polymeric printing plate. The relief of this plate wasthe mirror image of the design to be printed. The solution was thentransferred to from this plate to the metallised side of a black PETfilm with a thickness of 12 μm which was metallised with aluminium.After evaporation of the toluene with a fan, the film was then passedunder a UV lamp which polymerised the liquid crystal material leaving asolid film.

When viewed without a polariser the printed areas are apparent becauseof the change in surface gloss in the printed areas. When viewed througha linear polariser a deep blue colour is seen at 45° intervals as thepolariser is rotated. In between these areas no colour is seen. Whenviewed through a circular polariser the printed areas show up as lightagainst a black (dark) background. The use of a clear lacquer tooverprint the entire printed surface makes the printed areas totallyinvisible until a polariser is used to view the sample.

EXAMPLE 2

The solution from example 1 was printed as in example 1 but using apaper substrate that had previously been printed with Iriodin® pigmentsAfflair 201 and Afflair 211 (commercially available from Merck KGaA,Darmstadt, Germany) in a nitrocellulose binder.

When viewed without a polarising device the printed areas arediscernible because of their different surface gloss. When viewedthrough a linear polariser. When viewed through a circular polariser theprinted areas become clearly visible against a black background. The useof a lacquer to overprint the entire printed surface makes the printedareas totally invisible until a polariser is used to view the sample.

1. A method of preparing a birefringent marking comprising: printing apolymerisable liquid crystal material directly onto at least one surfaceof a reflective substrate; and polymerising the liquid crystal material,whereby a birefringent marking is formed on said reflective substrate,wherein said polymerizable liquid crystal material is printed onto thereflective substrate by screen printing, offset printing, dry offsetprinting, reel-to-reel printing, letter press printing, gravureprinting, rotogravure printing, flexographic printing, intaglioprinting, pad printing, heat-seal printing, ink-jet printing, orprinting by means of a stamp or printing plate, and printing of saidpolymerizable liquid crystal material onto the reflective substrateinduces or enhances spontaneous alignment of the polymerizable liquidcrystal material on said reflective substrate.
 2. A method according toclaim 1, wherein the liquid crystal material is a nematic or smecticliquid crystal material.
 3. A method according to claim 1, wherein thesubstrate comprises at least one metallic or metallised layer.
 4. Amethod according to claim 3, wherein the metal is selected fromaluminium, gold and copper.
 5. A method according to claim 1, whereinthe substrate comprises at least one layer of reflective pigments.
 6. Amethod according to claim 5, wherein the reflective pigments areselected from interference or pearlescent pigments and liquid crystalpigments.
 7. A method according to claim 1, wherein the liquid crystalmaterial comprises at least one compound which induces and/or enhancesplanar alignment.
 8. A method according to claim 7, wherein the compoundinducing and/or enhancing a planar alignment is a surfactant.
 9. Amethod according to claim 1, wherein the polymerised liquid crystalmaterial has a splayed structure.
 10. A method according to claim 1,wherein the polymerised liquid crystal material has a planar structure.11. A birefringent marking obtainable by a method according to claim 1.12. In a method of applying a decorative, security, authentification oridentification marking to an item, the improvement wherein said markingis a birefringent marking prepared according to claim
 1. 13. A securityauthentification or identification marking, thread or device comprisingat least one birefringent marking prepared according to claim
 10. 14. Ina document of value, a hot stamping foil, a reflective foil, or anoptical data storage device, the improvement wherein said document ofvalue, hot stamping foil, reflective foil, or optical data storagedevice has at least one birefringent marking according to claim
 11. 15.A document of value, a hot stamping foil, a reflective foil, or anoptical data storage device comprising at least one security,authentification or identification marking, thread or device accordingto claim
 13. 16. A method according to claim 2, wherein the polymerisedliquid crystal material has a planar structure.
 17. A security,authentification, or identification marking, thread or device comprisingat least one birefringent marking prepared according to claim
 16. 18. Amethod according to claim 1, wherein said birefringent marking isprepared separately on said at least one surface of said reflectivesubstrate; and then said marking and reflective substrate are applied toa document of value.
 19. A method according to claim 2, wherein saidbirefringent marking is prepared separately on said at least one surfaceof said reflective substrate; and then said marking and reflectivesubstrate are applied to a document of value.
 20. A method according toclaim 1, wherein said polymerizable liquid crystal material comprises aliquid crystal material and a solvent.
 21. A method according to claim1, wherein said polymerizable liquid crystal material further comprisesa polymeric binder or one or more monomers capable of forming apolymeric binder.
 22. A method according to claim 1, wherein saidpolymerizable liquid crystal material does not containing a binder. 23.A method according to claim 8, wherein said surfactant is a fluorocarbonsurfactant.
 24. A method according to claim 7, wherein the amount ofsaid compound in said polymerizable liquid crystal material 0.01-5weight %.
 25. A method according to claim 24, wherein said fluorocarbonssurfactant is of formula I:C_(n)F_(2n+1)SO₂N(C₂H₅)(CH₂CH₂O)_(x)CH₃  I wherein n is an integer from4 to 12, and x is an integer from 5 to
 15. 26. A method according toclaim 1, wherein said birefringent marking has a birefringence in therange from 0.1 to 0.3 and a thickness from 0.5 to 20 μm.
 27. A methodaccording to claim 1, wherein said birefringent marking furthercomprises one or more further layers applied onto said birefringentmarking.
 28. A method according to claim 27, wherein said one or morefurther layers are selected from a protecting layer, a support layer, anadhesive layer, a reflecting layer, an optical retardation layer, acolor filter, a polarizer, or combinations thereof.
 29. A methodaccording to claim 2, wherein said polymerizable liquid crystal materialfurther comprises a surfactant, said polymerizable liquid crystalmaterial is printed onto discrete regions of said reflective substrate,and said reflective substrate is a metallized or metal substrate.
 30. Amethod according to claim 29, wherein said liquid crystal material is anematic liquid crystal material.
 31. A method according to claim 2,wherein said polymerizable liquid crystal material further comprises asurfactant, said polymerizable liquid crystal material is printed ontodiscrete regions of a paper substrate that is covered by a layer ofinterference pigments dispersed in a transparent binder.
 32. A methodaccording to claim 31, wherein said liquid crystal material is a nematicliquid crystal material.
 33. A method according to claim 1, wherein saidbirefringent marking is invisible under unpolarized light and is visiblewhen viewed through a polariser.